Relation between the Electronic Properties of Regioregular Donor-Acceptor Terpolymers and Their Binary Copolymers

By analyzing the optical band gap and energy levels of seven different regioregular terpolymers in which two different electron-rich donor moieties are alternating with a common electron-deficient acceptor unit along the backbone, we establish a direct correlation with the properties of the corresponding binary copolymers in which one donor and one acceptor are combined. For this study, we use diketopyrrolopyrrole as the common acceptor and different π-conjugated aromatic oligomers as donors. We find that the optical band gap and frontier orbital energies of the terpolymers are the arithmetic average of those of the parent copolymers with remarkable accuracy. The same relationship is also found for the open-circuit voltage of the bulk heterojunction solar cells made with the ter- and copolymers in combination with [6,6]-phenyl-C₇₁-butyric acid methyl ester. Comparison of these findings with data in the literature suggests that this is a universal rule that can be used as a tool when designing new π-conjugated polymers. The experimental results are supported by a semiempirical quantum chemical model that accurately describes the energy levels of the terpolymers after parametrization on the energy levels of the copolymers and also provides a theoretical explanation for the observed arithmetic relations.

Conjugated Donor-Acceptor Terpolymers Toward High-Efficiency Polymer Solar Cells

The development of conjugated alternating donor-acceptor (D-A) copolymers with various electron-rich and electron-deficient units in polymer backbones has boosted the power conversion efficiency (PCE) over 17% for polymer solar cells (PSCs) over the past two decades. However, further enhancements in PCEs for PSCs are still imperative to compensate their imperfect stability for fulfilling practical applications. Meanwhile development of these alternating D-A copolymers is highly demanding in creative design and syntheses of novel D and/or A monomers. In this regard, when being possible to adopt an existing monomer unit as a third component from its libraries, either a D' unit or an A' moiety, to the parent D-A type polymer backbones to afford conjugated D-A terpolymers, it will give a facile and cost-effective method to improve their light absorption and tune energy levels and also interchain packing synergistically. Moreover, the rationally controlled stoichiometry for these components in such terpolymers also provides access for further fine-tuning these factors, thus resulting in high-performance PSCs. Herein, based on their unique features, the recent progress of conjugated D-A terpolymers for efficient PSCs is reviewed and it is discussed how these factors influence their photovoltaic performance, for providing useful guidelines to design new terpolymers toward high-efficiency PSCs.

Regulating Molecular Aggregations of Polymers via Ternary Copolymerization Strategy for Efficient Solar Cells

For many high-performance photovoltaic materials in polymer solar cells (PSCs), the active layers usually need to be spin-coated at high temperature due to the strong intermolecular aggregation of donor polymers, which is unfavorable in device repeatability and large-scale PSC printing. In this work, we adopted a ternary copolymerization strategy to regulate polymer solubility and molecular aggregation. A series of D-A₁-D-A₂ random polymers based on different acceptors, strong electron-withdrawing unit ester substituted thieno[3,4-b]thiophene (TT-E), and highly planar dithiazole linked TT-E (DTzTT) were constructed to realize the regulation of molecular aggregation and simplification of device fabrication. The results showed that as the relative proportion of TT-E segment in the backbone increased, the absorption evidently red-shifted with a gradually decreased aggregation in solution, eventually leading to the active layers that can be fabricated at low temperature. Furthermore, due to the excellent phase separation and low recombination, the optimized solar cells based on the terpolymer P1 containing 30% of TT-E segment exhibit high power conversion efficiency (PCE) of 9.09% with a significantly enhanced fill factor up to 72.86%. Encouragingly, the photovoltaic performance is insensitive to the fabrication temperature of the active layer, and it still could maintain high PCE of 8.82%, even at room temperature. This work not only develops the highly efficient photovoltaic materials for low temperature processed PSCs through ternary copolymerization strategy but also preliminarily constructs the relationship between aggregation and photovoltaic performance.